1. Field of the Invention
The present invention relates to an optical disc recording and reproducing apparatus preferable when used for a writable optical disc such as a write once optical disc (CD-R) and a rewritable optical disc (CD-RW), and a method for reading information.
Specifically, in the present invention, a photodetector detects a return light of a light beam applied to an optical disc in such a manner that a target pre-format address and recorded information are distinguished from interference information by the pre-format addresses adjacent to the target pre-format address at its inner and outer peripheral sides. With this arrangement, the target pre-format address and the recorded information can be read from the optical disc with excellent reproducibility, and the crosstalk between adjacent pregrooves can be prevented, even if the light beam is applied across adjacent information recording layers in accordance with the increase in a storage density of the optical disc.
2. Description of the Related Art
In recent years, as information media for editing and recording audio information in such a manner as to suit the preferences of listeners, writable optical discs such as write once optical discs (CD-R) and rewritable optical discs (CD-RW) standardized in ISO/IEC13490-1 are increasingly used. In each of these optical discs 10, as shown in FIG. 1A, grooves 1 for guiding a light beam (hereinafter, referred to as pregrooves) are formed, and a positioning method referred to as a tracking servo is employed. The tracking servo is a mechanism where pits and projections constituting lands 2 formed on both sides of each pregroove 1 are detected to decide the position of an optical pickup, so that the desired pre-format address is accurately irradiated with a laser beam.
Sloped surfaces of the land, which coincide to side surfaces of the pregroove 1, are for med with a slight wobble in the form of sine wave in-phase with each other as shown in FIG. 1B. The wobble signal indicating the wobble component has been subjected to FM modulation. In the wobble signal, time axis information which indicates the position on the optical disc 10, and a recommended value of the power of the laser beam optimum for recording are encoded.
This time axis information is referred to as MTIP (Absolute Time In Pregroove) information, and is written as an absolute time in the signal recording region (i.e. a program region) of the optical disc 10 along a direction from the starting point at its inner peripheral side toward its outer peripheral side. The ATIP information is written in the processing of the CD-R an d CD-RW. The ATIP information is written in the optical disc 10 such as CD-R and CD-RW in a modulation mode referred to as bi-phase modulation mode. In this mode, a baseband modulation is conducted where, depending on whether the waveform of the last pulse signal constituting the error correction information of the immediately preceding pre-format address ends at a high level or low level, the pre-format address in the next frame is connected without being inverted, or the pre-format address is inverted and connected.
The ATIP information is written, for example, in the side surfaces Us, Ut of the pregroove 1 between two lands 2a, 2b shown in FIG. 1B. That is, the ATIP information is written in the side surface Us of the land 2a located at the inner peripheral side of the pregroove 1 in which data is recorded, and in the side surface Ut of the land 2b located at the outer peripheral side of the pregroove 1 in such a manner that the ATIP information written in the side surface Us is synchronous to the ATIP information written in the side surface Ut.
Therefore, the ATIP information can be read as a wobble signal by detecting the reflected light from the main spot 4 on the wobble formed in the lands 2a, 2b by two light receiving elements split in the direction of track. The broken double line circles in FIG. 1B show spot diameters of the light beam applied to the optical disc having a standard density. The two-dot long and two short dashes line circles show a light beam of which irradiation diameter is relatively enlarged as the pregrooves are formed so as to be closer to each other in accordance with the increase in a storage density of the optical disc 10. This problem arises because there is a limitation on enlarging the spot diameters of the light beam.
The wobble signal is produced in such a manner that its center frequency becomes 22.05 kHz for example, when the optical disc 10 is rotated at a standard velocity (CLV) of a compact disc (CD). One sector of the ATIP information is constituted in such a manner as to coincide to one data sector after the signal is recorded. Therefore, at the time of recording information, the pregrooves 1 are irradiated with a light beam with a predetermined intensity and information is written therein while the sector of the ATIP information is synchronous with the data sector.
In a conventional method, when a signal is recorded in the optical disc 10, as shown by a broken line in FIG. 1B for example, a pit 61 is formed at a main spot 4 created in a three-beam method, and in this state, the return light from the main spot 4 is split and received in the photodetecting section 71 shown in FIG. 2. In the photodetecting section 71, a light receiving signal A from the light receiving element PD1 such as a photodiode which constitutes a four split photodetector 711, and a light receiving signal D from the light receiving element PD4 similar to the light receiving element PD1 are added to each other to produce an addition signal A+D. At the same time, a light receiving signal B from the light receiving element PD2, and a light receiving signal C from the light receiving element PD3 are added to each other to produce an addition signal B+C. In the photodetecting section 71, the latter addition signal B+C is subtracted from the former addition signal A+D to obtain a wobble signal.
In the above-described method, furthermore, the light receiving signals A to D from the four light receiving elements PD1 to PD4 in the photodetector 711 are also added to each other to produce an RF signal (A+B+C+D). At the same time, a tracking servo is controlled by use of a light receiving signal E from the light receiving element PD5, a light receiving signal F from the light receiving element PD6 of the two split photodetector 712, and a light receiving signal G from the light receiving element PD7, and a light receiving signal H from the light receiving element PD8 of the two split photodetector 713.
When the return light from the main spot 4 is employed, there arises a problem that the degree of reflection is changed in the process of forming the pit 61, and the amount of light of the return light from the main spot 4 is fluctuated. In an attempt to avoid such a problem, the signal levels of the light receiving signals A to H from the photodetecting section 71 are adjusted by use of a high speed AGC, so as to eliminate the influence of the fluctuation of the amount of light. Alternatively, the ratio between the addition value of the light receiving signal A from the light receiving element PD1 and the light receiving signal D from the light receiving element PD4, and the addition value of the light receiving signal B from the light receiving element PD2 and the light receiving signal C from the light receiving element PD3 by use of a divider, so as to eliminate the influence of the fluctuation of the amount of light. In the state where the influence of the fluctuation of the amount of light is eliminated in these manners, a wobble component is taken out to obtain time axis information and the like. Furthermore, in the information reading method according to the conventional example, time axis information and the like is obtained by sampling and holding an RF signal when the power of the laser beam is lowered from the signal writing level to the signal reading level.
However, if information is read based on the pre-format addresses which are written in the bi-phase modulation mode as it is without conducting any processing in the recent situation where the lands 2a, 2b are arranged at narrower pitches (hereinafter, also referred to as track pitches in some cases) P in accordance with the increase in the storage density of the optical disc 10 (hereinafter, referred to as a disc-shaped record medium in some cases), the light beam of which irradiation diameter is relatively enlarged as the pregrooves are formed so as to be closer to each other is applied across the adjacent pregrooves (see FIG. 1B).
In other words, even if the main spot 4 is in the shape of a complete circle, when the spot diameter is larger than the track pitches P, a leakage from the adjacent track occurs. In this case, there is much fear that a crosstalk is generated. The crosstalk causes a phase modulation to create jitter, and as a result, the signal-noise (S/N) ratio is lowered.
As a result, when the bi-phase signal constituting the target pre-format address at the reading point is superposed onto the bi-phase signal of the other pre-format address adjacent to the target pre-format address, there arises a problem that the phase difference therebetween is large, and larger jitter is created as the period of the high-level is shorter.
In addition, if it is attempted to obtain time axis information by use of the return light from the main spot 4 while forming the pit 61 in the main spot 4, a high speed AGC and a divider are required, and the structure of the apparatus becomes complicated. When the signal is written at a speed higher than a standard speed, the light amount of return light is fluctuated by the influence of the formation of pit, even if the power of the laser beam is lowered from the signal writing level to the signal reading level by sampling and holding the signal. In this case, there is a fear that the time axis information cannot be read accurately.
The present invention has been conducted to solve the above-described problems of the prior arts, and the objective thereof is to provide an optical disc recording and reproducing apparatus from which the target pre-format address and the recorded information can be read with excellent reproducibility, as well as being capable of preventing the occurrence of crosstalk in adjacent pregrooves, even if a light beam is applied across adjacent information recording layers in accordance with the increase in the storage density of the optical disc, and a method for reading information.
The above-described problems can be solved by an optical disc apparatus for recording information and reproducing the recorded information in an optical disc having addresses formed beforehand, and the information is to be recorded into the optical disc based on the addresses. The optical disc apparatus includes: a light output device for emitting a light beam to the optical disc; and a photodetector for detecting a return light which is reflected by the optical disc and contains information to be read, in such a manner that a target address and the recorded information are distinguished from interference information which is owing to addresses adjacent to the target address at its inner and outer peripheral sides.
According to an optical disc apparatus of the present invention, when the information which has been recorded in the optical disc based on the addresses is read therefrom, a light beam is applied from the light output device to the optical disc. The photodetector detects a return light of the light beam which reflects the information to be read from the optical disc in such a manner that the target address and the recorded information are distinguished from the interference information which is owing to the addresses adjacent to the target address at its inner and outer peripheral sides.
For example, four light receiving elements arranged vertically and horizontally in a center area around the light receiving axis of the photodetector detect the component of the return light which reflects the target pre-format address and the recorded information. On the other hand, the other four light receiving elements, two of which are arranged horizontally above the four light receiving elements, and the rest two of which are arranged horizontally below the four light receiving elements, detect the component of the return light which reflects the interference information by the pre-format addresses adjacent to the target pre-format address at its inner and outer peripheral sides.
The output from the photodetector is input into the signal generator. The signal generator produces an RF signal and a wobble signal based on the output from the four light receiving elements arranged vertically and horizontally in a center area around the light receiving axis of the photodetector. In addition, the signal generator produces a tracking error signal or a focus error signal based on the output from the eight light receiving elements.
Accordingly, the interference information by the pre-format addresses adjacent to the target pre-format address at its inner and outer peripheral sides is removed from the information read from the optical disc, and whereby only the recorded information of the target pre-format address can be taken out. As a result, even if the light beam is applied across the information recording layers adjacent to each other in accordance with the increase in the recording density of the optical disc, the occurrence of crosstalk can be prevented without damaging the function of tracking error detection and the function of focus error detection.
According to a method for reading information of the present invention, information is read from an optical disc having addresses formed beforehand, and the information is recorded in the optical disc based on the addresses. The method includes the steps of: irradiating the optical disc with a light beam; and detecting a return light reflected by the optical disc in such a manner that a component of the return light which reflects a target address and the recorded information are distinguished from a component of the return light which reflects interference information which is owing to addresses adjacent to the target address at its inner and outer peripheral side.
According to the method for reading information of the present invention, the interference information by the pre-format addresses adjacent to the target pre-format address at its inner and outer peripheral side is removed from the information read from the optical disc, and whereby only the recorded information of the target pre-format address can be taken out. As a result, even if the light beam is applied across the information recording layers adjacent to each other in accordance with the increase in the recording density of the optical disc, the occurrence of crosstalk can be prevented without damaging the function of tracking error detection and the function of focus error detection.